This invention relates to pipe and fittings and, more particularly, but not by way of limitation, the invention relates to double-containment piping systems used to contain leakage of hazardous fluids.
The use of dual containment piping systems in which an inner carrier pipe is coaxially placed within an outer containment pipe is a known and accepted commercial practice. Typically such systems have been found in the nuclear, oil and gas, petroleum refining, and chemical processing industries. The carrier pipe is used to transport hazardous or toxic fluid while the containment pipe is used to contain leakage from the carrier pipe. It is also known to provide leakage detectors and drainage systems within the annulus between the carrier and containment pipes.
One of the problems uniquely associated with double-containment pipe is created by the thermal expansion and/or contraction of the inner carrier pipe relative to the outer containment pipe. More specifically, the problem is restraining the contraction and expansion while providing a system which is relatively inexpensive in cost, installation, and maintenance.
Prior dual containment and concentric piping systems have addressed this problem by either providing restraints between the containment pipe and carrier pipe in the piping runs between fittings as well as between the inner carrier housing and the outer containment housing of the fittings, which greatly increases the cost of manufacturing and installing the systems; or by using restraint/socket-type restraint fittings having the restraining mechanism integral with a socket-type end connection for the containment housing of the fitting. Because of the structural complexity of the integral socket and restraining mechanism and the need for relatively close dimensional tolerances to assure proper fit and alignment of the integral socket and restraining mechanism with the fitting and with the desired orientation of the pipe runs connecting to the socket, the restraint/socket-type fittings need to be molded or cast, and are normally press-molded, i.e., because of the structural complexity and close dimensional tolerances the manufacturing alternative of hand laying up or hand machining the restraint/socket-type fitting is so labor intensive it is not economically or practically viable. Because the restraint/socket-type fittings are press-molded, the dimensions of the socket are fixed to the size of the mold and therefore limited to uses in which the O.D. (outer diameter) of the pipe spool is sufficiently controlled to seat and seal properly in the socket. The O.D. of fiber-reinforced thermoset resin ("thermoset") pipe cannot be controlled sufficiently to assure proper mating with a press-molded socket in pipe having an I.D. (internal diameter) greater than 14 inches. This is because thermoset pipe must be filament wound in sizes above 14 inch I.D. and the O.D. cannot be accurately controlled in the filament winding process (as opposed to pipe of 14 inch I.D. and below, which may be centrifugally cast and in which the O.D. is precisely controlled). Socket-type thermoset fittings may be press-molded in sizes of 14 inch I.D. and below, and may therefore be economically produced. However, in sizes above 14 inch I.D. the pressure rating of press-molded sockets is not reliable and therefore the socket must be hand made ("hand laid up"), and therefore the restraint/socket-type fitting is not suitable in sizes above 14 inch I.D. Similarly, in some industries, such as the pulp and paper industry, it is believed that the pressure rating of press-molded fittings of any size is not sufficiently reliable and hand made fittings are used exclusively. For the preceding reasons, prior to the present invention double-containment piping systems utilizing restraint fittings had not been designed or installed in sizes greater than 14 inch I.D. and the only double-containment pipe systems utilizing restraint fittings in sizes up to 14 inch I.D. utilized the fitting having a containment housing socket and integral restraint which must be press-molded to be of practical use.
Therefore, there is a need for a double-containment pipe fitting and system having a restraint-type fitting which may be used with all sizes of pipe; which may be used with both cast and filament wound thermoset resin pipe; which may be used with double-containment piping systems of virtually any material; which provides restraint against expansion, contraction, and other end loadings on the fittings exclusive of restraint in the pipe runs; which eliminates the need for expansion and contraction restraints in double containment pipe runs (or spools); which is relatively inexpensive to manufacture and install; which may be assembled using only adhesive connectors; and which may be economically manufactured and installed using socket-type connectors which are not press-molded.